In commonly assigned U.S. Pat. No. 4,821,572, a multi-axis angular rate sensor is disclosed. This device includes a plurality of accelerometers mounted on two frame members and is configured to allow the frame members to rotate about a common axis. A drive mechanism comprising first and second C-shaped electromagnetic coils and associated pole pieces is used to counter-rotate the frame members about the common axis without transmitting a reaction force to a supporting base. As the electromagnetic coils are alternately and sequentially energized with an electric current, the frame members are driven to rotatably dither back and forth in opposite directions.
Several problems are associated with the drive mechanism used in the multi-axis rate sensor disclosed in the above-referenced patent. Although the frame members rotate only a few degrees in each direction, the first and second electromagnetic coils are energized by current supplied through leads that are continually flexed as a result of the dither motion of the device. Eventually, even the most flexible conductors available may work-harden and break. Since the coils are attached adjacent the periphery of the frame members, the mass and rotational inertia of the frame members are substantially increased by the addition of the coils--even though an important design goal for this device was to minimize these parameters.
Conventional direct current (dc) electromagnetic motors capable of developing the torque required to drive the multi-axis rate sensor are comparatively bulky. Their bulk is necessary to accommodate permanent magnets, ferrous metal flux linkage members, and pole pieces that focus the magnetic flux across air gaps in the motor. A rotor in a conventional dc motor typically includes armature windings that are energized with electrical current conveyed through brushes, which produce undesirable radio frequency (RF) noise and are subject to wear. The mass of such a rotor and its inertia prevent it from quickly stopping and reversing direction. Accordingly, a conventional dc motor is not usable to drive the multi-axis rate sensor described above. Such motors are neither sufficiently compact, nor do they include a rotor that is sufficiently low in mass and inertia to rapidly rotate back and forth through a small incremental angle at the required rate. Since prior art dc motors are not easily adapted to this application, it has been necessary to develop a new motor to drive the multi-axis rate sensor.
To meet this need, a new compact torque motor, which provides bi-directional, limited angle, reactionless torque was developed for driving the multi-axis rate sensor. This motor is described in a copending, commonly assigned patent application, Ser. No. 07/369,373, filed on June 21, 1989 now U.S. Pat. No. 4,968,909. The motor includes an X-shaped core and two pole pieces that are disposed at opposite sides of the core. Two pairs of opposed legs on the core thus define transverse core sections. Electromagnetic coils are formed on the legs of each core section, and when the coils on the two core sections are alternately energized, the resulting magnetic force attracts tabs that are disposed on the ends of the pole pieces, causing the pole pieces to counter-rotate back and forth about a central axis. Because the pole pieces are lightweight, rotational inertia of the motor is low and its efficiency is high.
Further improvements in the design of the multi-axis rate sensor have created a need for an even more compact torque motor than that just described. Ideally, a suitable torque motor should make use of one or more components required for other aspects of the rate sensor in order to reduce the total number of parts and to minimize the mass, size, and cost of the device. For example, the new rate sensor design uses a permanent magnet and a plurality of electromagnetic coils to monitor the velocity of the accelerometers as the frame members in which they are mounted dither back and forth. Since the permanent magnet is already in use for this purpose, a torque motor that uses the same permanent magnet to drive the rotors and the accelerometers to dither back and forth achieves the desired reduction in parts count and cost.
Accordingly, it is an object of the present invention to provide a dc motor that has very lightweight rotors, yet generates relatively high torque. A further object is to provide a motor that produces bi-directional torque simply by changing the direction in which electrical current flows through the motor, where the torque is independent of the extent of rotation for rotor rotation up to about .+-.1.5 degrees. These and other objects and advantages of the present invention will be apparent from the attached drawings and the Description of the Preferred Embodiments that follow.